This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
In the oil and gas industry, seismic prospecting techniques commonly are used to aid in the search for and evaluation of subterranean hydrocarbon deposits. A seismic prospecting operation consists of three separate stages: data acquisition, data processing, and data interpretation. Success of the operation depends on accurate and geologically consistent completion of all three stages.
In the data acquisition stage, a seismic source is used to generate an elastic wave, which propagates in the form of a seismic signal into the subsurface structure of the earth. The elastic wave is at least partially reflected from a boundary between one or more layers, generally due to contrasts in layer properties. More specifically, adjacent layers in the subsurface have different physical properties due to changes in lithology and fluids, and thus may present different densities and velocities affecting the strength of the reflected elastic wave. The contrast in these properties at the boundaries between adjacent layers causes the reflections that may then be recorded by a receiver as a seismic trace and processed for imaging the subsurface. The processed seismic trace can be modeled as a convolution of the Earth's impulse response with the seismic signal produced by the source.
Generally, elastic wave propagation through earth subsurface may create a complex seismic response with different reflection and transmission modes. Two types of elastic waves may be generated during reflection and/or transmission at a boundary between layers, which are compressional waves (P-waves) and shear waves (S-waves). P-waves propagate with a higher velocity through the Earth than S-waves, generally by a factor of two to four and therefore arrive at detectors earlier than S-waves. P-waves are compressional waves that are longitudinally polarized. In other words, the particle motion is aligned with the direction of the wave propagation through the layers. By contrast, the S-waves are shear waves in which the particle motion is perpendicular to the direction of wave propagation. S-waves are generally grouped into two classifications, SV-waves, in which the particle motion is aligned in the plane of reflection, and SH-waves, in which the particle motion is perpendicular to the plane of reflection. Typically, S-wave reflections are generated when a P wave is incident at a non-zero angle of incidence to the reflecting surface.
For example for a P-wave source, data collected from the arrival of P-waves are generally referred to as PP mode data, because these events involve seismic energy that is emitted as a P-wave and that arrives at the receiver as a P-wave. Similarly also for a P-wave source, data arising from the arrival of S-waves are generally referred to as PS mode data, because these events involve energy that is emitted as a P-wave and that undergoes mode-conversion to an S-wave upon reflection and so arrives at the receiver as an S-wave. Vertical and horizontal components typically measured in 3-component seismic data may generally be a combination of PP and PS modes. PP mode data may contribute more strongly in vertical components of the acquired seismic data, whereas PS mode data may contribute more strongly in the horizontal component of the acquired seismic data.
In multi-component seismic acquisition, response of subsurface is measured by multi-component receivers. During the data processing stage, the recorded seismic signals are separated into dominant PP and PS modes, refined and enhanced using a variety of procedures that depend on the nature of the geologic structure being investigated and on the characteristics of the raw acquired seismic data. Processed seismic data contains useful subsurface information in a frequency range defined by the “seismic bandwidth” of the data which depends upon seismic sources and receivers used in the seismic survey. Seismic bandwidth may be enhanced during seismic processing. In general, the purpose of the data processing stage is to produce a data representation, or image, of the subsurface from the acquired seismic data for use during the data interpretation stage.
The results of the data interpretation stage may be used to determine the general geologic structure of a subsurface region, to estimate rock properties, to locate potential hydrocarbon reservoirs, and/or to guide the development of a previously discovered reservoir. The accuracy of the image obtained by analyzing the seismic signals may be limited by the amount of data obtained for a specific site. Accordingly, using both PP mode data and PS mode data may improve the accuracy of the image. However, the difference in P-wave and S-wave propagation velocities, among others, necessitates aligning PP and PS mode reflection data and makes combining data collected in the different modes challenging in a joint PP/PS analysis. In seismic literature, PP/PS alignment is often referred to as data registration. When the terms “alignment” or “align” are used herein in the sense of aligning two sets of data, it is noted that the terms “registration” or “register” could alternatively be used instead. Previous techniques to align PP and PS data (Fomel et al, “Multicomponent Seismic Data Registration by Least Squares,” SEG Intl Exposition and Seventy-Third Annual Meeting (2003); Van Dok et al., “Event Registration and Vp/Vs Correlation Analysis in 4C Processing,” SEG Intl Exposition and Seventy-Third Annual Meeting (2003); and U.S. Pat. No. 7,082,368 to Nickel (2006)) are developed using generic PP mode and PS mode reflection data volumes. However, because of the differences in phase and amplitude of PP and PS reflections, all forms (for example, near stack, full stack, angles stacks) of PP mode and PS mode data may not fulfill the assumption implicit in the alignment techniques. Many alignment techniques may force a match the between PP mode and PS mode data even where such a match may not be theoretically justified. This forced matching can lead to time alignment errors with serious effects in joint PP/PS inversion and geologic interpretation of PP/PS data. To alleviate the phase problem, seismic envelope data is often used for alignment, but envelope data lack the seismic resolution critical for accurate registration of PP and PS data. Responding to these needs, the present invention discloses use of certain derivative products of PP and PS seismic data that are theoretically expected to be in phase and retain the seismic resolution in the data.